I’m on a team of developers working on synthetic device drivers for
FreeBSD on Hyper-V. In late March, I briefly mentioned (on this list)
that we're trying to get our drivers included with the FreeBSD
distribution. Also noted were some necessary patches to the ATA driver.
The changes are necessary to achieve significant performance gains by
replacing the native ATA driver with our synthetic storage driver
when when Hyper-V is detected. Alexander Motin, the maintainer of the
ATA code-base, however, expressed some concerns about making these
modifications that are so specific to Hyper-V and the AMD64 build. We
understand his concerns and have subsequently removed these patches from
our vendor branch. Our drivers are now completely independent and
require no changes to the kernel or other drivers. The only touch-point
is the common makefile for modules/drivers.

Removing our ATA patches, on the other hand, results in a huge
performance loss. This is because the root file system is managed by the
ATA driver, which is emulated under Hyper-V. Furthermore, there may be
other native drivers that we wish to replace with synthetic ones at boot
time (e.g., mouse driver, video driver), but, there appears to be no
easy way to do this.Therefore, we would like to work with the developer
community to come up with better solution that would improve support for
synthetic drivers at boot-time.
One possible solution:
(1) Move the call to init_param1() (in sys/kern/subr_parm.c), which is
used for hypervisor detection, to an earlier point in the boot process.
Presently, it appears to be called after the ATA driver is selected,
which is too late in the boot process. (This was discovered after some
testing with the ATA driver.) Therefore, before the bus drivers and
native controllers are detected and selected, discovery of a host
hypervisor should be done first.
(2) Extend the boot process to dynamically insert and prioritize
synthetic drivers over native ones. Presently, FreeBSD supports the
selection of generic drivers for a given device, but these can be
overridden when a more specific driver is available. This priority
scheme could be extended by first detecting the presence of a
hypervisor, then allowing this priority mechanism to override the native
drivers with their synthetic cousins (i.e., we only want to override a
native driver when a specific hypervisor is detected and synthetic
drivers for it have been configured). This could be arranged in a
separate, table-driven, input file or perhaps, by extending the existing
driver/module configuration file with additional mappings of native to
synthetic drivers.
(3) Upgrade the init_param1() function (in sys/kern/subr_parm.c) to use
the more recent approach to hypervisor detection. This approach uses the
CPU-identify functions to retrieve a unique signature consisting of a
fixed string of ASCII characters. This was done on Linux about five
years. For backward compatibility, however, the existing logic would be
retained, but augmented with this new approach. It would also be
conditionally added only for x86/AMD64 builds.
When reviewing the changes we're not looking at a lot of lines of
code.The difficultly lies, however, in where the changes are made.
Obviously, we need to work closely with the committers responsible for
the kernel modules we're looking to touch. Retrospectively, these
upgrades only bring FreeBSD up to the same level of hypervisor support
already present in Linux.There are other issues that we would also like
to discuss, but the improvements described here are on the top of the list.
We look forward to working more closely with the FreeBSD community and
welcome your remarks and feedback on this proposal.

I am sorry, I don't really understand what is your proposition, so I
just describe my vision of the solution here, hoping it will be useful.

You are right that FreeBSD has priority mechanism to select best driver
for each device. That mechanism should be sufficient to prevent default
OS driver from attaching to emulated ATA controller when Hyper-V
environment is detected and paravirtual drivers are installed. To do
that you should just create a very small dummy driver for PCI bus,
implementing single probe() method. That probe() method should in your
preferable way check that Hyper-V environment is active and match PCI ID
of the device against the list of used by Hyper-V for emulated ATA
controller. If both conditions match, the probe() method should return
highest priority value to override any other ATA driver in system for
this specific device. After that the dummy driver should just have empty
attach() and detach() routines, not doing anything with the emulated
device, relying on paravitrual driver to handle the disks.

Since the dummy driver can be made machine-dependent, you may use there
any VM detection mechanisms you like, even direct MSR access. Dummy
driver can be made completely independent from the rest of the system,
so it can even be loaded as kernel module without requiring any base
system modifications. If you still prefer it for some reason (for
example to not do some complicated checks multiple times in multiple
drivers), you may move Hyper-V detection logic out of that driver to
some single place in base system. But you don't need to touch OS-native
ATA driver in any way.